Expanded fiber is a substance made from fibrous material having a fibrillar ultrastructure, wherein the fibrous material has been processed in such a way as to cause fibrils to separate from, or become disassociated from, the fibrous material ultrastructure. Alternatively, expanded fiber can be considered as cellulosic fibrous material which has been expanded from a fibrous form to a fibrillar form. Expanded fiber from natural, cellulosic fibers is of particular interest herein.
Cellulosic fibers are multi-component ultrastructures made from cellulose polymers. Lignin, pentosans and other components known in the art may also be present. The cellulose polymers are aggregated laterally to form threadlike structures called microfibrils. Microfibrils are reported to have diameters of about 10-20 nm, and are observable with an electron microscope. Microfibrils frequently exist in the form of small bundles known as macrofibrils. Macrofibrils can be characterized as a plurality of microfibrils which are laterally aggregated to form a threadlike structure which is larger in diameter than a microfibril, but substantially smaller than a cellulosic fiber. In general, a cellulosic fiber is made up of a relatively thin primary wall, and a relatively thick secondary wall. The primary wall, a thin, net-like covering located at the outer surface of the fiber, is principally formed from microfibrils. The bulk of the fiber wall, ie, the secondary wall, is formed from a combination of microfibrils and macrofibrils. See Pulp and Paper Manufacture, Vol. 1, Properties of Fibrous Raw Materials and Their Preparation For Pulping, ed. by Dr. Michael Kocurek, Chapter VI, "Ultrastructure and Chemistry", pp 35-44, published jointly by Canadian Pulp and Paper Industry (Montreal) and Technical Association of the Pulp and Paper Industry (Atlanta), 3rd ed., 1983, incorporated herein by reference. The cellulosic fiber walls constitute the ultrastructure of the cellulosic fiber. Microfibrils and macrofibrils shall hereinafter be collectively referred to as "fibrils." Expanded fiber from cellulosic fibers thus refers to fibrils which have been substantially separated from or disassociated from a cellulosic fiber ultrastructure. Fibrous material in this condition shall hereinafter be referred to as being in "fibrillar" form.
Expanded fiber has a high proportion of surface area relative to conventional fibrous material. Expanded fiber from cellulosic fiber is particularly characterized by high binding ability, and high gellability. Expanded fiber has application in a variety of areas including strength additives, thickeners, extenders and carriers in a variety of structural products, foods, drugs, cosmetics, paints, and other industrial and chemical applications.
Production of expanded fiber, of any type, from fibrous material having a fibrillar ultrastructure involves expansion of the fibrous material from a primarily fibrous form to, at least, a partially fibrillar form. One method for producing expanded fiber from cellulosic, fibrous material is disclosed in U.S. Pat. No. 4,483,743, Turbak, et al., issued Nov. 20, 1984. Expanded fiber, referred to therein as microfibrillated cellulose, is produced by passing a liquid suspension of cellulose fibers through a small diameter orifice, in which the suspension is subjected to a pressure drop of at least 3000 psig and a high velocity shearing action, followed by a high velocity decelerating impact. Passage of the suspension through the orifice is repeated until a substantially stable suspension is obtained. While this method produces expanded fiber having desirable absorption and settling volume properties, it is not believed to provide efficiencies of scale which may be critical for competitiveness with competing cellulosic substances at mass production levels. Therefore, it is desirable to provide an alternative method of making expanded fiber.
Other methods in the paper industry having been proposed to increase the level of fibrillation conventionally observed for pulped, cellulosic fibers. For example, beating and additional refining of pulp in excess of the level conventionally practiced in order to provide a commercially saleable product are well known to to increase fibrillation. However, beating and refining as practiced in the cellulose fiber industry are relatively inefficient processes. Large amounts of energy are expansion and fibrillation. In relatively low amounts of fiber expansion and fibrillation. In these processes, the fiber is abraded to form a fiber having a "fuzzy" character, while the fiber walls, and hence the ultrastructure, are retained substantially intact. Beating and refining, generally implemented by abrasion and impacting of suspended fibers by entrapment between a rotor and stator, have been found to be of extremely limited utility for producing expanded fiber due to the prolonged period of fiber treatment necessary to achieve levels of fibrillation significant for the manufacture of expanded fiber. Another disadvantage of fibrillation by conventional beating and refining apparatuses is that a high level of wear would be incurred upon the apparatus surfaces.
Another type of cellulosic material made from cellulosic fibers is particulate cellulose. Various forms of particulate cellulose have been available for a number of years in the cellulose industry. Particulate cellulose is mechanically disintegrated, purified cellulosic fibrous material. As its name indicates, particulate cellulose exists in a particulate or powdered state rather than a fibrillated state. The particulate state is a result of mechanical processing which breaks a relatively large number of chemical bonds within the cellulosic fibrils and fibrous ultrastructure. Methods for producing particulate cellulose include conventional ball milling; and include, to produce particularly finely powdered cellulose, sonic pulverization with a modified ball mill. Although particulate cellulose is useful for a variety of applications including utilization as food additives, thickeners and extenders, it does not provide as high a degree of binding and gellability as obtained in connection with the use of expanded fiber.
Other types of finely divided celluloses are known which involve the use of chemical treatments in their manufacture, such as acid hydrolysis and mercerization. However, such additional chemical treatments lead to increased chemical and disposal costs and reduced yield of cellulosic product. One common form of cellulose made from chemically treated fibers is known as microcrystalline cellulose. The accessible amorphous regions of the fibers are chemically dissolved, leaving only the crystalline regions in the form of fine crystals. In addition to the disadvantages listed above, microcrystalline cellulose is also less reactive and absorptive than other finely divided cellulose forms.
It is therefore an object of this invention to provide a process for making expanded fiber from fibrous material having a fibrillar ultrastructure.
It is also an object of this invention to provide a process for making expanded fiber from cellulosic fibrous material without causing excessive cellulose chain degradation or cellulose dissolvation.
These objects, and other advantages that may be or become apparent to those skilled in the art, have been attained by the present invention which is described below.